In various recording materials including pressure-sensitive, heat-sensitive and light-sensitive materials, a leuco dye is frequently used as a color image forming substance. The leuco dye, which is also referred to as a redox dye, develops a color on contact with an acid color developer.
The leuco dye is generally contained in microcapsules which are dispersed in a recording material. In an image forming process, the microcapsules are broken by external energy such as pressure and/or heat so that the dye comes into contact with the acid color developer which has been arranged outside of the microcapsules in the recording material.
Examples of the leuco dye which develops a yellowish color are described in Japanese Patent Publication Nos. 45(1970)-4698, 50(1975)-24646, 51(1976)-27169 and 53(1978)-9127 (corresponding British Pat. No. 1421493), and Japanese Patent Provisional Publication No. 49(1969)-4480. However, these leuco dyes do not satisfy the requirement with respect to light fastness, that is, the developed color tends to fade from the image when it is irradiated with light.